1. Technical Field
The present invention is related to controlling a flow cell battery and, in particular, to thermal control of a flow cell battery.
2. Discussion of Related Art
Reduction-oxidation (redox) flow batteries store electrical energy in a chemical form, and subsequently dispense the stored energy in an electrical form via a spontaneous reverse redox reaction. A redox flow battery is an electrochemical storage device in which an electrolyte containing one or more dissolved electro-active species flows through a reactor cell where chemical energy is converted to electrical energy. Conversely, the discharged electrolyte can be flowed through a reactor cell such that electrical energy is converted to chemical energy. Electrolyte is stored externally, for example in tanks, and flowed through a set of cells where the electrochemical reaction takes place. Externally stored electrolytes can be flowed through the battery system by pumping, gravity feed, or by any other method of moving fluid through the system. The reaction in a flow battery is reversible; the electrolyte can be recharged without replacing the electroactive material. The energy capacity of a redox flow battery, therefore, is related to the total electrolyte volume (i.e., the size of the storage tank). The discharge time of a redox flow battery at full power also depends on electrolyte volume and can vary from several minutes to many days.
The minimal unit that performs the electrochemical energy conversion is generally called a “cell,” whether in the case of flow batteries, fuel cells, or secondary batteries. A device that integrates many such cells, coupled electrically in series and/or parallel to get higher current, voltage, or both, is generally called a “battery.” However, it is common to refer to any collection of coupled cells, including a single cell used on its own, as a battery. As such, a single cell can be referred to interchangeably as a “cell” or a “battery.”
Redox flow batteries can be utilized in many technologies that require the storage of electrical energy. For example, redox flow batteries can be utilized to store night-time electricity that is inexpensive to produce, and to subsequently provide electricity during peak demand when electricity is more expensive to produce or demand is beyond the capability of current production. Such batteries can also be utilized for storage of green energy (i.e., energy generated from renewable sources such as wind, solar, wave, or other non-conventional sources). Flow redox batteries can be utilized as uninterruptible power supplies in place of more expensive backup generators. Efficient methods of power storage can be used to construct devices having a built-in backup that mitigates the effects of power cuts or sudden power failures. Power storage devices can also reduce the impact of a failure in a generating station.
Other situations where uninterruptible power supplies can be of importance include, but are not limited to, buildings where uninterrupted power is critical, such as hospitals. Such batteries can also be utilized for providing an uninterruptible power supply in developing countries, many of which do not have reliable electrical power sources, resulting in intermittent power availability.
In a flow battery, the electrolyte is typically a multi-salt solution. The solubility of each salt in the presence of other salts depends upon the relative concentrations of each salt and the temperature. Usually, all the salts are soluble within a certain temperature range and precipitate outside of the range, resulting in damage to the flow battery system and reduced efficiency of the flow battery system.
The internal resistance of a flow battery depends upon the electrolyte solution temperature. Typically, as the electrolyte temperature increases, the internal resistance decreases and hence the efficiency of the system increases. Therefore, to operate the battery system efficiently, the flow battery system can be operated at a high temperature. However, in certain chemistries, one or more electrolyte salts may decompose in the solution at high temperatures, resulting in permanent damage to or temporary loss of efficiency in the system. Also at higher temperatures, parasitic reactions (e.g., H2 or O2 generation) may occur, which can offset the efficiency gains noted above.
Therefore, operation of a flow cell battery is optimal when the electrolyte temperature is maintained within a certain range (i.e., efficient operating range). A redox flow battery operates efficiently within a range of temperatures, for example, between about 30° C. and about 50° C. However, the flow battery may experience a thermal environment which varies widely over time. For example, temperatures may be hot during the day and cold at night, varying by tens of degrees over the course of a twenty-four hour period. Conventional flow batteries employ heaters and coolers to control the temperature of the electrolyte. However, these techniques result in additional equipment costs and higher operating costs. Hence, it is desirable to have a thermal control technique for maintaining electrolyte temperature which consumes minimal amounts of energy and which does not use heaters and/or coolers.